Riyad-us-Saliheen 1271
Ibn 'Umar (May Allah be pleased with them) reported: The Messenger of Allah (PBUH) said, "(The superstructure of) Islam is based on five (pillars), testifying the fact that La ilaha illallah wa anna Muhammad-ar-Rasul-ullah [there is no true god except Allah, and Muhammad ((PBUH)) is the Messenger of Allah], establishing As-Salat (the prayers), paying Zakat (poor due), the pilgrimage to the House [of Allah (Ka'bah)], and the Saum (fasting) during the month of Ramadan."
[Al-Bukhari and Muslim]
This Hadith has been mentioned earlier. Here it has been repeated in support of the obligation of the fourth pillar of Islam. Thus, Hajj is compulsory for a person who fulfills the qualifications required for it and has the means for it.